1. Field of the Invention
The present invention relates to a glass substrate for an information recording medium (hereinafter also referred to simply as “a glass substrate”), and more particularly to a glass substrate for use as a substrate of an information recording medium such as a magnetic disk, magneto-optical disk, DVD, or MD.
2. Description of the Prior Art
Conventionally, magnetic disks for use in stationary devices such as desk-top computers and servers typically have substrates made of aluminum alloy, and those for use in portable devices such as notebook computers and mobile computers typically have substrates made of glass. However, aluminum alloy is prone to deformation, and is not hard enough to offer satisfactory surface smoothness on the surfaces of a substrate after polishing. Moreover, when a head makes mechanical contact with a magnetic disk, the magnetic film is liable to exfoliate from the substrate. For these reasons, substrates made of glass, which offer satisfactory surface smoothness and high mechanical strength, are expected to be increasingly used in the future not only in portable devices but also in stationary devices and other home-use information devices.
One known type of glass substrate is those made of chemically strengthened glass, in which the alkali elements present near the surface of the substrate are replaced with other alkali elements in order to produce compression strain and thereby increase mechanical strength. In the manufacture of chemically strengthened glass, the process of chemical strengthening is typically performed after the process of polishing, and glass substrates that have undergone the chemical strengthening process are shipped intact as an end product without being subjected to any further process. This necessitates subjecting the glass substrates to polishing that attains flatness higher than is eventually required in order to allow for the normally unavoidable degradation of the their flatness as a result of the chemical process. Moreover, if the glass substrates develop deformation as a result of the chemical process, there is no choice but to discard them as defective. This makes it difficult to achieve a sufficiently high yield rate. To overcome these inconveniences, for example, Japanese Patent Application Laid-Open No. 2000-76652 proposes a technique of subjecting glass substrates first to a chemical strengthening process and then to a polishing process with a view to improving productivity and associated results.
However, according to the technique proposed in the publication mentioned above, although the strengthened layer on the main surface is removed by polishing in the polishing process performed after the chemical strengthening process, a predetermined thickness of the strengthened layer is left even after the polishing process. Thus, if the strengthened layer left after the polishing process is uneven, it degrades geometrical quality such as flatness. To prevent this, the top and bottom surfaces of glass substrates need to be polished uniformly, and this requires a high-precision polishing technique.